This invention relates to a high pressure fuel injection unit for an engine and more particularly to an improved accumulator type injector nozzle for engines.
A popular and very effective type of injection nozzle for injecting fuel to an internal combustion engine is the so called "accumulator type". With this type of injection nozzle, there is provided an accumulator chamber in which fuel is stored under pressure and which can communicate with the engine through a discharge port. An injection valve is mounted within the accumulator chamber and controls the opening and closing of the discharge port. There is also provided a control chamber that receives fuel under pressure from the accumulator chamber and the injection valve has a portion that is supported within this control chamber. As a result, the control chamber pressure acts on the injection valve and assists in maintaining it in a closed position. A control valve selectively relieves the pressure in the control chamber. When this occurs, the pressure of the fuel in the accumulator chamber will cause the injection valve to open and fuel to be discharged. With this type of system, it is also the practice to employ a biasing spring that acts on the injector valve so as to urge it to its closed position in cooperation with the fuel pressure in the control chamber.
Although this type of injection nozzle is very effective, it is difficult at times to provide good running at low engine speeds such as idle. The reason for this is that at low pressures under low speed operation the amount of fuel discharged will increase significantly for a given increase in fuel pressure. This may be understood best by reference to FIG. 1 which is a graphical view showing the fuel pressure in the accumulator chamber in relation to the amount of fuel discharged during four varying running conditions, indicated by the curves a, b, c and d. The curves a, b, c and d indicate respectively increasing engine speeds with the curve a being idle speed and the curve d being wide open throttle, maximum engine speed. It can be seen that the curve a is quite sharp. Hence a small difference in fuel pressure in the accumulator chamber will make a substantial difference in the amount of fuel injected.
It is very difficult to control adequately the pressure of the fuel in the accumulator chamber at low engine speeds. This is because the fuel pressure changes with a number of variables primarily at temperature, which effects the viscosity.
In order to reduce these fluctuations in amount of fuel discharge at low engine speeds in response to pressure variations, it has been proposed to increase the biasing preload of the spring that cooperates with the pressure in the control chamber to hold the injection valve in its closed position. Although an increased spring preload will lessen the slope of the curves shown in FIG. 1, this gives rise to another problem. That is, at high engine speeds when the actual time duration of opening of the injection nozzle is quite short, it may be impossible to inject sufficient fuel so as to achieve maximum performance. Therefore, the previously proposed systems have not been effective in solving the aforenoted problem while at the same time maintaining maximum engine output.
It has been proposed to offset these disadvantages through the use of an arrangement for varying the preload on the biasing spring which holds the injector valve in its closed position. Such an arrangement is shown in application Ser. no. 07/556,234 as invented by me, entitled ACCUMULATOR TYPE INJECTION NOZZLE, filed Jul. 20, 1990 and assigned to the assignee thereof. Such a device is effective in changing the shape of the delivery curves from those shown in FIG. 1 to the curves identified as a, `b,' c' and d` in FIG. 2. Although this construction provides a very efficient result, still further improvements are possible.
The reason for this may be best understood by reference to FIGS. 3 and 4 wherein fuel delivery rate curves with respect to time or crank angle at idle or low speed and at high speed, respectively, are shown by the curves a and d and indicated in broken lines. In these curves, the actual time the injection valve is open is indicated as either t (time) or 8 (crank angle). The time of energization of the actuating solenoid is indicated by the solid line curves identified by the reference characters TW. It will be noted that there is a lag, for readily apparent reasons, between the time when the solenoid is energized and the time when the injection valve is actually opened and also between the time when the solenoid is de-energized and the injection valve closes.
As may be readily seen from FIG. 3, the curve a provides a gradually increasing rate of fuel discharge up until a peak, which occurs after the solenoid has been de-energized, and then rapidly falls off as the valve closes. The problem with this type of injection is that the fuel is delivered primarily at the end of the injection cycle and this may not insure complete combustion nor good fuel economy.
At high speeds, on the other hand, the fuel delivery builds up rapidly to a peak which occurs at approximately the time the solenoid is de-energized and then falls rapidly. Said another way, the rate of peak fuel delivery is at approximately the mid-point of the time when the injection valve is opened. This type of fuel delivery tends to introduce too much fuel early in the combustion cycle and can give rise to knocking, pre-ignition and other engine related combustion noises, particularly in diesel engines. In addition, the type of fuel delivery provided by conventional nozzles as indicated by the curve d tends to reduce the maximum output of the engine for a given amount of fuel consumed.
It is, therefore, a principal object of this invention to provide an improved accumulator type injection nozzle.
It is a further object of this invention to provide an improved accumulator type injection nozzle that will provide good fuel control at low speeds even with pressure variations.
It is a further object of this invention to provide an improved accumulator type injection nozzle wherein the sensitivity of fuel discharge at low pressures is reduced without adversely effecting the amount of fuel which can be discharged at high pressures.
It is a further object of this invention to provide an improved arrangement for operating an accumulator type injection nozzle so as to cause smaller changes in fuel discharge with pressure changes under low pressure conditions than at high pressure conditions.
It is a further object of this invention to provide an improved accumulator type of fuel injection nozzle that will provide the introduction of greater amounts of fuel into the engine at the beginning of the injection cycle than conventional injectors at low engine speeds and loads.
It is another object of this invention to provide an improved accumulator type fuel injector that will supply the peak rate of fuel discharge into the engine at high speeds and high loads at the end of the injection cycle.